Patterning of the gastrointestinal tract during embryogenesis results in distinct morphologic and functional regions along the anterior-posterior (A-P) axis of the small intestine. The epithelial cells of the small intestine are derived from the gut endoderm cell lineage and function in the digestion and absorption of ingested nutrients with spatial differences in gene expression patterns along the A-P axis. Intestinal malformations and defects in epithelial cell function can often manifest as lethal diseases in humans. However, much remains to be learned about the genes that regulate gut formation. The overall goal of this proposal is to elucidate molecular mechanisms regulating gastrointestinal patterning, differentiation and function. Evidence suggests that Hox and Parahox functions establish important developmental axes in the embryo, including anterior-posterior fates in the gastrointestinal tract. Studies of mice have recently demonstrated a role for the Parahox member pancreatic duodenal homeobox-1 protein, PDX-1, in establishing and maintaining differentiation of the developing pancreas. However, the role of PDX-1 in small intestinal development has not been fully elucidated. Preliminary studies have shown that PDX-1 is capable of inducing and repressing intestine-specific genes. We hypothesize that PDX-1 is involved in establishing and maintaining epithelial cell gene expression patterns along the A-P axis of the small intestine through regulation of intestinal target gene promoters. Research objectives are therefore aimed at characterizing the role of PDX-1 in intestinal development and elucidating mechanisms involved in PDX-1 regulation of intestinal genes. In Aim 1, we will define the role of PDX-1 in patterning small intestinal development using mouse models with conditional duodenal PDX-1 inactivation and ectopic PDX-1 overexpression. In Aim 2, we will characterize gene expression profiles and identify novel PDX-1 intestinal target genes by utilizing microarray technology. In Aim 3, we will perform molecular experiments aimed at characterizing mechanisms involved in PDX-1 transcriptional regulation of intestinal target gene promoters. Our combination of in vitro and in vivo experimental approaches should elucidate roles for PDX-1 in patterning gastrointestinal development and will add to our understanding of vertebrate organogenesis. In addition, knowledge of mechanisms regulating gut epithelial gene expression may provide novel diagnostic, therapeutic or prognostic strategies for gastrointestinal disorders that result in loss or inappropriate gain of epithelial function.